Transmission of only a single carrier is supported in a Long Term Evolution (LTE) Rel-8 system, and thus a User Equipment (UE) measures a channel quality over only a single downlink carrier and feeds back a measurement result of Channel State Information (CSI). The CSI includes Rank Indicator (RI), Channel Quality Indicator (CQI), Precoding Matrix Indicator (PMI) and other information. A specific feedback can be performed periodically over a Physical Uplink Control Channel (PUCCH) or aperiodically over a Physical Uplink Shared Channel (PUSCH).
When the CSI is transmitted aperiodically over the PUSCH, the RI information is encoded separately, and the CQI and the PMI are encoded separately, and ACK/NACK information is also encoded separately, and all such uplink control information is multiplexed with uplink data as illustrated in FIG. 1. After multiplexing and interleaving, uplink data symbols arc mapped to physical resources by firstly mapping the CQI/PMI information and then mapping the uplink data onto the remaining resources. The ACK/NACK and the RI information is mapped by punching the uplink data so that the ACK/NACK is placed in four columns of symbols on both sides of reference signal in an order of ascending frequencies firstly in the time domain and then in the frequency domain and the RI information is mapped in four columns of symbols immediately next to the ACK/NACK in an order of ascending frequencies firstly in the time domain and then in the frequency domain.
In the LTE Rel-8 system, aperiodic transmission of CSI over a PUSCH is triggered by 1-bit CQI Request information in an uplink (UL) grant. If the UE receives the CQI Request bit which is set to 1, then the UE forms the relevant CSI information and inserts in a pre-configured feedback scheme over the PUSCH and transmits it; otherwise, no CSI is transmitted.
In order to support a larger system bandwidth in an LTE-advanced system than in the LTE system, resources of a plurality of LTE carriers (also referred to as component carriers) have to be connected together for use, particularly in two approaches:
1) A plurality of consecutive LTE carriers are aggregated to provide the LTE-A system with a larger transmission bandwidth; and
2) A plurality of inconsecutive LTE carriers are aggregated to provide the LTE-A system with a larger transmission bandwidth.
FIG. 3 illustrates an example of aggregation of a plurality of inconsecutive carriers.
An existing research trend of the standardization organization lies in such a commonly accepted idea of a conceived system with carrier aggregation that a design over each carrier is kept consistent with the LTE Rel-8 as much as possible to thereby ensure a user equipment of the LTE Rel-8 system can operate normally over each component carrier.
A research demand of the existing LTE-A system has been ascertained that aggregation of up to at most 5 component carriers can be supported and a UE can support concurrent transmission/reception of data over at most 5 component carriers.
A support of uplink and downlink asymmetric and symmetric carrier aggregation has been decided in the research on carrier aggregation. Asymmetric and symmetric carrier aggregation can be configured from the perspective of the system (that is, an uplink and downlink asymmetric or symmetric carrier configuration can be supported in the system) or allocated per UE (that is, a UE is allocated with an uplink and downlink asymmetric or symmetric carrier configuration).
Also a discussion has been made in the research on carrier aggregation about the issue on a pairing relationship between uplink and downlink carriers, where a downlink carrier can be paired with one or more uplink carriers and an uplink carrier can also be paired with one or more downlink carriers. From the perspective of the system, such a pairing relationship is as illustrated in FIG. 4A and FIG. 4B with an arrowed connection between an uplink carrier and a downlink carrier representing the presence of a pairing relationship between both of them, where a pair of downlink/uplink carriers with a linking relationship is referred to as a cell, and the correspondence relationship between the uplink and downlink carriers in each cell is broadcasted from the system to all the user equipments in the cell.
FIG. 5 illustrates a UE-specific uplink and downlink asymmetric or symmetric carrier configuration supported in the LTE-A Rel-10, where a UE1 is configured with an uplink and downlink symmetric carrier configuration, and linking relationships of paired carriers in two cells are the same as contents broadcasted in respective system messages. A UE2 is configured with an uplink and downlink asymmetric carrier configuration, and at this time an Uplink Component Carrier (UL CC) can belong to only one cell; and a linking relationship of a UL CC3 with a Downlink Component Carrier (DL CC) 3 applies to the UE, but a linking relationship thereof with a DL CC4 does not apply to the UE, where the DL CC4 is referred to a standalone Downlink Component Carrier (DL CC).
In another scenario, the numbers of uplink and downlink carriers are symmetric from the perspective of the system, and there is a one-to-one linking relationship; and a UE is allocated with asymmetric numbers of uplink and downlink carriers in the system as illustrated in FIG. 6, where the DL CC2 in the figure is also referred to a standalone DL CC, and also this configuration scenario has to be supported in the LTE-A Rel-10.
Following a discussion about the capability of an LTE-A Rel-10 user equipment, user equipments with an uplink and downlink symmetric aggregation capability and an uplink and downlink asymmetric aggregation capability will be present in the Rel-10, where each user equipment reports its own uplink and downlink carrier aggregation capability to a base station in an initial access procedure. The base station cannot configure downlink and uplink carriers beyond the uplink and downlink carrier aggregation capability of the user equipment.
Solutions to carrier activation/deactivation have been introduced in the LTE-A Rel-10, where activation and deactivation of a downlink carrier is triggered by a Media Access Control layer Control Element (MAC CE), and the UE measures and feeds back CSI only over an activated DL CC.
In the LTE Rel-8, a aperiodic CSI feedback is triggered by a 1-bit CQI Request information in a UL grant. In the discussion about the LTE-A, it is generally recognized that a 1-bit CQI Request in a UL grant continues the use of the design in the LTE Rel-8 without any extension or modification, and specific multi-carrier aperiodic CSI feedback schemes are as follows:
In a first scheme, aperiodic CSI information of a plurality of DL CCs are fed back concurrently;
The UE is triggered by a 1-bit CQI Request information in a UL grant to put aperiodic CSI information of all the activated DL CCs over a PUSCH scheduled by the UL grant for feedback together. This scheme is the simplest but has a drawback of a considerable feedback overhead and thus poses a considerable influence upon transmission performance over the PUSCH, especially when the UE is configured with a plurality of downlink component carriers (up to 5) and there is a small physical resource scheduled over the PUSCH.
In a second scheme, aperiodic CSI information of each DL CC is fed back separately, particularly in the following two schemes, and such schemes also have their corresponding restrictions although they avoid the problem of a feedback overhead resulting from CSI information of a plurality of DL CCs being fed back concurrently.
In a scheme 2-1, the UE is triggered by a 1-bit CQI Request information in a UL grant to put aperiodic CSI information, of each DL CC in which the UL grant is transmitted from the network, over a PUSCH scheduled by the UL grant for feedback.
This scheme has such drawbacks that: 1) for a UE configured with cross-carrier scheduling, e.g., the UE1 in FIG. 5, where a dotted arrow represents a configuration of uplink cross-carrier scheduling, that is, PUSCH transmission of both the UL CC1 and the UL CC2 is scheduled by a UL grant over the DL CC1, here no UL grant is transmitted over the DL CC2 as concluded so far, and a aperiodic CSI feedback of the DL CC2 cannot be triggered; and 2) no UL grant will be transmitted over a standalone downlink component carrier as concluded so far, and thus a aperiodic CSI feedback thereof cannot be triggered either.
In a scheme 2-2, the UE is triggered by a 1-bit CQI Request information in a UL grant to put aperiodic CSI information, of each DL CC having a system-level paired linking relationship with each UL CC scheduled by the UL grant, over a PUSCH scheduled by the UL grant for feedback.
This scheme has such a drawback that aperiodic CSI of a part of DL CCs of a UE configured with uplink and downlink carriers cannot be fed back, e.g., the DL CC4 of the UE2 in FIG. 5 and the DL CC2 of the UE3 in FIG. 6.